Some modern head-mounted display systems, such as those employed in connection with virtual reality (VR) or augmented reality (AR) systems, employ eye-tracking technology in order to enhance the operation of the overall system. Eye-tracking systems often use one or more light sources to project light outside the visible spectrum (e.g., light in the infrared (IR) and/or near-infrared (NIR) spectrum) onto one or both eyes of a viewer. An imaging subsystem or device may then capture and process the projected light that is reflected by the viewer's eyes to “track” (i.e., determine where the viewer is looking) the eyes of the viewer, such as by detecting the location of the viewer's pupils, detecting the shape of the viewer's corneas, and so on. Head-mounted display systems may use eye-tracking information for a variety of reasons. For example, a VR system may, upon identifying an area of a virtual image at which the viewer is currently gazing, sharpen that area of the image while slightly blurring other areas to simulate the actual depth of field (DOF) characteristics of the human eye.
In conventional head-mounted systems with eye-tracking technology, an optical element that passes visible light but reflects IR light, sometimes referred to as a “hot mirror,” may be disposed between a display device of the head-mounted display system and the eyes of the viewer at an angle relative to the display (e.g., at 45 degrees). In such systems, the optical element may allow light projected by the display device to reach the viewer's eyes while redirecting IR or NIR light reflected by the viewer's eyes to the eye-tracking subsystem, which may in turn enable the eye-tracking subsystem to be positioned outside of the viewer's field of view. Unfortunately, the size and/or relative angle of hot mirrors used in conventional head-mounted systems typically increase the distance between the viewer's eyes and the display, as well as the overall size of the system, potentially resulting in increased manufacturing and shipping costs and/or decreased comfort and other issues for end-users.